The U.S. Federal Emergency Management Agency (FEMA) determines an area’s risk for flooding for a variety of reasons. The City of Boston is vulnerable to floods not only because it is a coastal city, bordered by both a harbor and open ocean, but also because it is mostly built on fill.

FEMA released revised flood maps in 2013. These new maps updated the methodology and data inputs used to determine flood risk, as well as accounted for erosion and natural shifts in the city through wear and tear.

The number of people and businesses affected by the new maps was surprising. The City of Boston wanted to ensure the accuracy of the new maps released by FEMA, so they decided to hire an independent group to take a closer look. The Woods Hole Group was chosen to perform its own analysis of Boston’s flood risks.

During their analysis, the Woods Hole Group discovered that there was more detailed data that could have been used as a baseline. Another key finding was a difference in opinion about the type of wave modeling used. The wave model used in the original analysis was for an open ocean scenario. Because most waves in Boston come from the Boston Harbor, the open ocean model produced different results than what would actually occur within Boston’s closed harbor structure.

With the new information from the analysis, the Woods Hole Group proposed several changes to FEMA’s flood maps. On September 16, 2015, FEMA approved all the changes the Woods Hole Group had suggested to the flood maps. The new revised flood maps, which encompass much less of Boston than originally proposed and are the most scientifically accurate, will go into effect on March 16th, 2016.